In the present specification, the expression "effective refractive index", or simply "effective index", without further indications, is used to indicate the group effective index.
Group refractive index n.sub.g is the parameter determining the propagation speed of a light pulse, including a certain range of wavelengths centered about a nominal value .lambda., in an optically transparent medium. The group refractive index depends on phase refractive index n according to relation n.sub.g =n-.lambda.dn/d.lambda.. In an optical guide, such as a fiber, the refractive index is replaced by the corresponding effective index. Determination of the group effective index is then necessary whenever knowledge of the propagation speed of a pulse in a guide is desired.
A typical application may be in instruments based on backscattering measurements for locating faults in optical telecommunication fibers or cables in service.
It is known that to perform such measurements a radiation pulse is launched into the fiber under test, the backscattered radiation is analyzed, and the presence of a possible fault and its position are recognized from the delay with which the pulse echo is received, once pulse propagation speed is known. In general, the instruments directly supply distance information, and to this end the operator must each time calibrate the instrument scale, by loading the value of the effective index of the fiber into the instrument. It is evident that the accuracy with which that value is known determines in turn the accuracy of the distance information obtained.
Generally, effective index is determined by fiber manufacturers during fabrication. According to the most widely used technique, a light pulse is launched into a fiber span, the length of which has been previously measured and the end faces of which have been treated so as to increase their reflectivity, and the directly transmitted pulse as well as pulses having undergone one or more reflections at each end face are collected from the end face opposite to the launching end. Speed, and hence effective index, is derived from the propagation time difference. Use of a rather long fiber span (a few meters at least) is required for such measurement.
That technique, even though conceptually simple, has a number of drawbacks. More particularly, it is difficult to measure the span length with the desired accuracy, because a stretch of a meter or more long is not easy to handle: Thus, effective index is generally determined on a single fiber out of a batch, and the value obtained is considered valid for all fibers in the batch, even though a certain degree of tolerance with respect to a nominal value is admitted also for the effective index, like for any other optical fiber characteristic. For that reason, the measurement results is somewhat inaccurate, and that error adds to the systematic errors introduced by the apparatus exploiting the effective index value measured.